1. Field of the Invention
This invention is directed to a method for accessing an underground pressurized gas main for retirement or replacement of a saddle-mounted gas service pipe that is connected to the gas main while the gas main is under operating pressure.
2. Description of the Related Art
Underground pipes can be accessed through a hole that, at ground level, is small relative to the distance that the underground pipe, such as a gas main is located beneath the surface of the ground. This is sometimes referred to as “keyhole” excavation.
In most cases, the ground is first excavated, preferably by a vacuum excavation system, so that the gas main becomes uncovered, particularly at a location along the gas main where a gas service pipe, for example a saddle-mounted gas service pipe, is connected to the gas main, and through which the gas service pipe is connected to a household, a business, etc., for providing gas service thereto. U.S. Pat. Nos. 6,618,966; 6,669,406; 7,018,137 and 7,241,084 represent U.S. patents that facilitate “keyhole” excavation, for use in excavation situations in which the depth of the pipe to be accessed underground is substantially greater than the size of the excavation opening at ground level. The complete disclosures of said patents are herein incorporated by reference.
It is also known in the art of accessing underground pressurized gas mains that when a gas service pipe is to be removed from the gas main, either for purposes of retiring that gas service, or to replace the gas service with an alternative service, to do the same in such a way that avoids a rapid dissipation of gas from the gas main, an explosion, or the like.
A rapid dissemination of gas from the gas main is considered a gas “blow”, which is highly undesirable in that it may damage personnel, equipment, and in worse cases, can create a fireball or explosion.
Consequently it is desirable to work on the gas main for removal or retirement of the connection of a gas service pipe to the gas main under controlled conditions that minimize escape of gas from the gas main while such work is ongoing.